1. Technical Field
The present general inventive concept relates to an ink composition, and more particularly, to an ink composition that includes a coloring agent with a modified surface to improve stability, thus having uniform particle size and stronger adhesion to paper.
2. Description of the Related Art
A coloring agent is generally defined as a material that exhibits a unique color by selectively absorbing or reflecting visible light. Coloring agents can be divided into dyes and pigments.
A dye is a coloring agent that is printed onto and absorbed by a material, such as fiber, leather, fur, paper, or the like, so that the printed material has excellent color fastness to solar cleaning, rubfastness, and the like. On the other hand, a pigment is a micro particle having a coloring agent, and is not directly absorbed by but rather adhered to a surface of a material using a physical method, such as adhesion or the like, so that the printed material has a unique color. At this time, the unique color of the coloring agent, such as a pigment or a dye, corresponds to a portion of the visible light of 3000 to 7000 Å selectively reflected or absorbed by the coloring agent.
In a method of manufacturing a pigment-type ink, a pigment including a coloring agent must be dispersed in a solvent. As a result, the dispersion of the pigment is mainly dependent on a size distribution and distribution stability of the pigment particles. In addition, in order to prolong a shelf life span of the pigment-type ink, aggregation or precipitation of the pigment particles must be prevented over a long time and under varying ambient conditions, such as temperature.
Typically, a pigment-type black ink contains carbon black as the coloring agent. The carbon black can be dispersed in an ink solvent using a dispersing agent. U.S. Pat. No. 3,687,887 discloses a method of dispersing carbon black in a solvent using a stylene-maleic acid anhydride copolymer, and U.S. Pat. No. 4,697,794 discloses a method of dispersing carbon black in a solvent using a hydrophilic polymer dispersing agent including an ionic group, such as carboxylic acid, sulfonic acid, sulfate, and the like.
The carbon black can also be dispersed using a commercially available dispersing agent, which can be easily obtained. However, when a water-soluble dispersing agent is used to disperse the carbon black pigment in a water-soluble medium, an excessive amount of the water-soluble dispersing agent is required to effectively disperse the pigment, because the water-soluble dispersing agent is physically adsorbed to a surface of the carbon black. The amount of water-soluble dispersing agent needed is comparable to the amount of pigment to be dispersed, and results in poor dispersion efficiency. Also, even though the dispersing agent is bound to the pigment, the bonding is ineffective and unstable, and thus, stable dispersion cannot be obtained. As a result, ink including the dispersing agent cannot be stored for a long time, and the aggregation or precipitation of non-soluble particles may occur in the ink.
A block copolymer and a graft copolymer can be used as a dispersing agent. In this case, however, manufacturing costs are high. When a common surfactant is used, excessive bubbles can be generated in a dispersing operation.
In order to overcome these problems resulting from the use of the dispersing agent and the surfactant, the surface of the carbon black can be modified for dispersion. According to U.S. Pat. Nos. 5,630,868 and 5,672,198, the carbon black pigment can be transformed into a self-dispersible coloring agent by introducing an ionic group to the surface of the carbon black thorough a reaction between the carbon black pigment and a diazonium salt. In order to produce a self-dispersible coloring agent for an ink composition, in particular, a self-dispersible pigment, a cation group, such as a tertiary ammonium group (—NR3+), a tertiary phosphonium group (—PR3+) and a tertiary sulfonium group (—SR2+) where R is a C1-C10 alkyl group or a hydrogen atom can be bound to the surface of the carbon black. The self-dispersible coloring agent can also be made by introducing an anionic component, such as —COOM, —SO3M, —SO2NH2, —PO3HM, —PO3M2 where M is a hydrogen atom, an alkali metal, ammonium, and an organic ammonium, to the surface of the carbon black (U.S. Pat. Nos. 5,630,868 and 5,672,198).
Examples of commercially available self-dispersing pigments include pigments obtained from CABOT Inc. and Orient Inc.
An ink composition using a self-dispersible coloring agent has excellent dispersing properties without needing a dispersing agent, due to the ionic group of the self-dispersible coloring agent. However, the water resistance of the ink composition is decreased due to its high ionic properties with respect to water. In order to overcome this problem, a non-ionic group can be included in the ink composition in addition to the ionic group.